It is well known that the same temperature will produce a different sensation of heat or cold at different levels of relative humidity, air speed and thermal radiation. Most people will feel comfortable at 80 degrees Fahrenheit and 20% relative humidity in calm air at given thermal radiation, but at the same temperature, air speed, and thermal radiation nearly everyone will feel uncomfortable with humidity increased to 90%. The single thermal comfort index, called effective temperature is being used to represent reaction of most people to a combined effect of air temperature, humidity, thermal radiation and air speed. With increased air movement the discomfort associated with air temperature, humidity or thermal radiation can be greatly reduced.
Many types of temperature responsive fan speed control circuits are currently known. However, they do not allow the setting of a known effective temperature and they do not provide a constant level of effective temperature sensed by the human body when air temperature, relative humidity, thermal radiation and distance between the fan and the human body are changing. U.S. Pat. No. 4,734,012 discloses a fan speed control circuit where triac conduction angle is varying in response to air temperature. But the issue of setting the known effective temperature and providing a constant level of effective temperature when even only air temperature is changing, is not addressed. To provide a constant level of thermal sensation when air temperature is changing, even at constant level of relative humidity, thermal radiation and distance between the fan and the human body, the fan speed has to be varied in a specific non-linear way to achieve required air velocity. FIG. 3 shows the required air speed to maintain an effective temperature, at a constant level of 74 degrees Fahrenheit when air temperature is changing and the humidity and external thermal radiation are constant. Graphs showing influence of air temperature, relative humidity, air speed and mean radiant temperature on thermal comfort can be found in ASHRE HANBOOK OF FUNDAMENTALS, published by the American Society of Heating and Air-Conditioning Engineers.
Besides effective temperature, many other thermal comfort indexes are currently being used to represent the combined effect of air temperature, humidity, thermal radiation and air speed on human thermal comfort. Some of them, like sensible temperature, apparent temperature or wet-bulb globe temperature (WGBT) are expressed in degrees Fahrenheit. New thermal comfort index can be created by testing the reaction of people to the combined effect of air temperature, relative humidity, air speed and mean radiant temperature on thermal comfort. From the test data a lookup table can be created and used with a microprocessor. This new thermal comfort index does not necessarily have to be expressd in degrees Fahrenheit. Similar to a description commonly used on comfort charts, just three levels of thermal comfort can be used: cool, comfortable and warm. This approach will greatly reduce the amount of required memory for the microprocessor lookup table.
Though the present invention is being described by using effective temperature as a preferred thermal comfort index, it is not dependent on any specific thermal comfort index representing the relation between air temperature, relative humidity, air speed and a mean radiant temperature. As a result, the description of this invention, presented below is also valid when the effective temperature is replaced with a thermal comfort index and an adjusted effective temperature (ETa) is replaced by an adjusted thermal comfort index (TCIa).
The human body maintains normal internal temperature at about 98 degrees Fahrenheit by the physiological process of thermoregulation in response to external factors of air temperature, humidity, thermal radiation and air velocity. For the body to maintain normal temperature, the sum of the following: heat gain from body metabolism, heat loss by sweat evaporation, heat gain or loss by convection, heat gain or loss by conduction and heat gain or loss by radiation must equal zero. If the sum is positive, internal temperature will start rising and thermal injury to the body is possible. The human body looses heat mainly by the process of convection, evaporation and radiation. At air temperature above 90 degrees Fahrenheit the human body stays cool mainly by the process of sweat evaporation from the skin. At 80 degrees Fahrenheit heat loss due to convection and radiation is about equal to the heat loss due to evaporation. The process of heat transfer by evaporation, convection and conduction is influenced by air movement. By varying air speed in a specific way it is possible to create a comfortable environment over a wide range of air temperature, humidity and thermal radiation. In the application of the present invention where the human body may gain heat from external thermal radiation, e.g. from the sun, sensing the level of this radiation is important so that required air speed could be delivered. In most air conditioning systems the air velocity is below 20 feet per minute in order to avoid drafts of cold air. But when circulating air has temperature equal to the room air temperature, much higher air velocity is acceptable. Even in an office environment where high air speed is not acceptable, the effective temperature can be reduced by about 4 degrees Fahrenheit without disturbing papers. For workers performing physical labor much higher air speed is acceptable and higher reduction of effective temperature is possible.
Effective temperature is a complex function of air temperature, relative humidity, air velocity and thermal radiation. Required effective temperature can be achieved by many different combinations of the above variables. By changing only the air speed, the effective temperature can be maintained over a wide range of other variables. However, there is an upper limit for the air velocity to provide cooling. For example, at high air temperature and low humidity the human body may not be able to sweat fast enough to match the rate of evaporation and too high rate of air movement may actually reduce the rate of human body heat loss. Also, above certain air temperature, which depends on humidity and thermal radiation, the heating effect of the moving air is greater than the cooling effect.
When the required effective temperature cannot be achieved by varying the air speed, three warning indicators are provided to show the reason. Low cost versions of the fan controller for the present invention are possible with the use of a microprocessor.
The present invention can be used for table fans, ceiling fans, fans in cars or buses and the like. This invention will allow the keeping of higher temperature in air coditioned spaces and at the same time will allow indivdual persons to control effective temperature to meet their paticular requirements.
The main object of the present invention is to allow the setting of, and maintenance of a desired level of effective temperature produced by a fan when air temperature, humidity, thermal radiation and distance between the fan and the human body are changing.
It is another object of this invention to provide warning indication when the fan is running at maximum speed and is unable to produce set effective temperature because even higher fan speed is required to deliver required air velocity.
It is a further object of this invention to provide warning indication when the combined effect of air temperature, humidity, thermal radiation and air speed has reached a point at which any further increase of air speed will not result in additional effective air temperature reduction.
It is a further object of this invention to provide warning indication when the fan is unable to maintain the set effective temperature because existing effective air temperature is below the set effective temperature.
These and other objects of the present invention will be best understood from reading the following description of specific embodiments in connection with the accompanying drawings.